Analysis and Design of AC Amplifier Circuits Constructed by Integrated Operational Amplifiers

introduction

Integrated operational amplifiers (abbreviated as integrated op amps or op amps) are widely used in electronic circuits . Most typical application circuits of op amps are analyzed in detail and in depth in various electronic technology textbooks, but many textbooks do not introduce the AC signal amplification circuit composed of integrated op amps. Although some textbooks introduce it, the introduction is simple and the analysis is not comprehensive. The AC amplification circuit composed of integrated op amps has the advantages of simple circuits, no debugging, and low failure rate. Nowadays, the AC amplification circuits in many electronic products are generally composed of op amps. A comprehensive analysis of the composition and parameter calculation of various AC amplification circuits composed of integrated op amps is helpful for the inspection and maintenance of such circuits, as well as the reasonable design and use of AC amplification circuits composed of integrated op amps.

1 Analysis of operational amplifier AC amplifier circuit

1.1 Op amp AC amplifier circuit using dual power supply

In order to make the op amp have zero output when the input is zero, the internal circuit of the op amp is designed according to the requirements of using dual power supplies. The op amp AC amplifier circuit uses dual power supplies to increase the dynamic range.

1.1.1 Dual-power supply common-phase input AC amplifier circuit

Figure 1 is a common-phase input AC amplifier circuit using dual power supplies. The two sets of power supply voltages VCC and VEE are equal. C1 and C2 are input and output coupling capacitors ; R1 forms a DC path at the common-phase input of the op amp, and the internal differential tube obtains the necessary input bias current ; RF introduces DC and AC negative feedback, and forms a DC path at the inverting input of the integrated op amp, and the internal differential tube obtains the necessary input bias current; because C isolates the DC, the DC forms full feedback, and the AC is shunted through R and C to form AC partial feedback, which is voltage series negative feedback. After introducing DC full feedback and AC partial feedback, the DC voltage gain can still be very small (1 times) when the AC voltage gain is large, thereby avoiding the saturation of the op amp caused by the input offset current.

When there is no signal input, the voltage V0 at the output of the op amp is ≈ 0V, and the output voltage U0 of the AC amplifier circuit is 0V; when an AC signal is input, the voltage V0 at the output of the op amp varies between -VEE and +VCC, and the amplified AC signal is output through C2, and the amplitude of the output voltage uo is approximately VCC (VCC=VEE). After introducing deep voltage series negative feedback, the voltage gain of the amplifier circuit is the input resistance of the amplifier circuit Ri=R1／／γif. γif is the closed-loop input resistance after the op amp introduces series negative feedback. γif is very large, so Ri=R1／γif≈R1; the output resistance of the amplifier circuit R0=γof≈0, γof is the closed-loop output resistance after the op amp introduces voltage negative feedback, and rof is very small.

1.1.2 Dual-power supply inverting input AC amplifier circuit

Figure 2 is an inverting input AC amplifier circuit using dual power supplies. The two power supply voltages VCC and VEE are equal. RF introduces DC and AC negative feedback, C1 isolates DC, so that DC forms full feedback, and AC is shunted through R and C1 to form AC partial feedback, which is voltage parallel negative feedback. In order to reduce the zero drift caused by the input bias current of the op amp, R1=RF can be selected. After introducing deep voltage parallel negative feedback, the voltage gain of the amplifier circuit is Because the inverting input terminal of the op amp is "virtual ground", the input resistance of the amplifier circuit Ri≈R; the output resistance of the amplifier circuit R0=r0f≈0.

1.2 Op amp AC amplifier circuit using single power supply

In the AC amplifier circuit using capacitor coupling, in static state, when the DC voltage at the output of the integrated operational amplifier is not zero, due to the DC isolation effect of the output coupling capacitor, the voltage output by the amplifier circuit is still zero. Therefore, the integrated operational amplifier does not need to meet the requirement of zero output when zero input. Therefore, the integrated operational amplifier can be powered by a single power supply, with its -VEE terminal connected to the "ground" (i.e., the negative pole of the DC power supply), and the +Vcc terminal of the integrated operational amplifier connected to the positive pole of the DC power supply. At this time, the voltage V0 at the output of the operational amplifier can only vary between 0 and +Vcc. In the AC amplifier circuit of the operational amplifier powered by a single power supply, in order to prevent the amplified AC signal from being distorted, in static state, the voltage V0 at the output of the operational amplifier is generally set between 0 and +Vcc, that is, V0=+Vcc/2. This can obtain a larger dynamic range; in dynamic state, V0 increases to a value close to +Vcc and decreases to a value close to 0V on the basis of the +Vcc/2 value, and the amplitude of the output voltage uo is approximately Vcc/2. See the original manuscript for Figure 3

1.2.1 Single-supply common-phase input AC amplifier circuit

Figure 3 is a common-mode input AC amplifier circuit using a single power supply. The power supply Vcc is divided by R1 and R2, so that the potential of the common-mode input terminal of the op amp Since C blocks DC, RF introduces DC full negative feedback. Therefore, in static state, the voltage V0 of the op amp output terminal is V-≈V+=+Vcc/2; C passes AC, so RF introduces AC partial negative feedback, which is voltage series negative feedback. The voltage gain of the amplifier circuit is

Input of amplifier circuit Resistance Ri=R1／R2／rif≈R1／R2,

The output resistance of the amplifier circuit is R0=r0f≈0.

1.2.2 Single-power supply inverting input AC amplifier circuit

Figure 4 is an inverting input AC amplifier circuit using a single power supply. The power supply Vcc is divided by R1 and R2 to make the potential of the op amp's in-phase input terminal equal to V+. In order to avoid the interference of the power supply ripple voltage on the V+ potential, a filter capacitor C3 can be connected in parallel at both ends of R2 to eliminate resonance; since C1 blocks DC, RF introduces DC full negative feedback. Therefore, in static state, the voltage V0 at the output of the op amp = V-≈V+=+Vcc/2; C1 passes AC, so that RF introduces AC partial negative feedback, which is voltage parallel negative feedback. The voltage gain of the amplifier circuit is the input resistance Ri≈R of the amplifier circuit, and the output resistance R0=r0f≈0 of the amplifier circuit.

2 Design of operational amplifier AC amplifier circuit

When designing a single-stage op amp AC amplifier circuit,

(1) Select an integrated operational amplifier that can meet the requirements of use. In an AC amplifier circuit using capacitor coupling, since the capacitor blocks DC, the temperature drift voltage output by the AC amplifier circuit is very small. Therefore, the requirements for the drift performance of the integrated operational amplifier can be reduced, and the selection of the integrated operational amplifier should be mainly considered from the aspects of conversion rate, gain bandwidth, noise, etc. For pulse signals, wide-band AC signals, and video signals, an integrated operational amplifier with a higher conversion rate and a gain bandwidth of at least 10 times the highest operating frequency should be selected. High-speed and low-noise integrated operational amplifiers are often used in audio AC amplifier circuits that have relatively high requirements for sound quality, such as the dual operational amplifier 4558, NE5532, etc.

2) Determine whether to use dual power supply or single power supply. When the use conditions permit, the op amp AC amplifier circuit should use dual power supply as much as possible to increase the linear dynamic range. When the integrated op amp uses dual power supply, the positive and negative power supply voltages should generally be symmetrical. The power supply voltage should not exceed the use limit, and the power supply filtering should be good. In order to eliminate the low-frequency self-excitation caused by the internal resistance of the power supply, 0.01~0.1 μF capacitors are often added between the positive and negative power supply wiring and the ground for decoupling. When using a single power supply, the potential of the op amp's in-phase input terminal should be less than the maximum common-mode input voltage of the op amp.

(3) Determine whether the input signal is a common-phase input or a reverse-phase input. If the input resistance of the amplifier circuit is required to be relatively large, a common-phase input AC amplifier circuit should be used. This is because the increase in the input resistance of the reverse-phase input AC amplifier circuit will affect the voltage gain. From the relevant calculation formulas in Figure 2 or Figure 4, it can be seen that when the input resistance of the reverse-phase input AC amplifier circuit is increased, the voltage gain of the circuit will decrease, and the voltage gain will also be affected by the internal resistance of the signal source. Therefore, when designing a reverse-phase input AC amplifier circuit, it is sometimes difficult to take both the input resistance and the voltage gain into account. When using the common-phase input AC amplifier circuit of Figure 1 or Figure 3, the bias resistance value of R1 in Figure 1 is appropriately increased, or the voltage divider resistance values ​​of R1 and R2 in Figure 3 are appropriately increased, the input resistance of the amplifier circuit can be increased without affecting the voltage gain. In addition, in order to effectively increase the input resistance of the amplifier circuit of Figure 3, some improvements can be made to the circuit, and the improved circuit is shown in Figure 5.

The input resistance Ri of the amplifier circuit is ≈ R3. When the value of R3 is large, the value of the input resistance Ri of the amplifier circuit is large. Therefore, the input resistance of the amplifier circuit is significantly increased.

(4) Determine the voltage gain of the AC amplifier circuit. The voltage gain Au of a single-stage op amp AC amplifier circuit should not exceed 100 times (40dB). Too high a voltage gain will not only reduce the passband of the amplifier circuit, but also easily induce high-frequency noise or generate self-oscillation. If you want to get an amplifier with a relatively large gain, you can use a two-stage equal-gain op amp circuit or a multi-stage equal-gain op amp circuit to achieve it.

(5) Determine the resistance value in the AC amplifier circuit. In general applications, the resistance value is between 1 and 100 kΩ. In high-speed applications, the resistance value is between 100 Ω and 1 kΩ, but it will increase power consumption. In portable designs, the resistance value is between 1 and 10 MΩ, but it will increase system noise. First set the resistance value of the op amp's reverse input terminal R in the figure, and then estimate the value of the feedback resistor RF based on the voltage gain calculation formula of the relevant circuit. It is best to use metal film resistors to reduce internal noise.

(6) Determine the capacitance value in the amplifier circuit. The size of the signal coupling capacitor determines the low-frequency characteristics of the amplifier circuit. Select the coupling capacitor value according to the frequency of the AC amplifier circuit signal. If the amplified signal is a low-frequency AC signal, such as an audio signal, the coupling capacitor value can be selected between 1 and 22 μF; if the amplified signal is a high-frequency AC signal, the coupling capacitor value can be selected between 1000pF and 0.1 μF. The DC blocking capacitor value for the in-phase input AC amplifier circuit that introduces DC full feedback is estimated by the formula C=1/20πfR. In the formula, f is the lowest frequency of the input signal. The lowest frequency of the audio signal is 20Hz. When R≥1kΩ, after the above formula estimation, selecting C=100 μF can meet the requirements. The filter capacitor value is selected between 100 and 1000 μF.

3 Conclusion

When deep AC negative feedback is introduced, the voltage gain, input resistance, etc. of the op amp AC amplifier circuit are only related to the external resistance of the integrated op amp. Therefore, compared with the triode AC amplifier circuit, the design of the op amp AC amplifier circuit is more convenient and the consistency of the circuit parameters is better.